This invention is directed toward the field of helical scan tape recorders. More specifically, the invention is an apparatus and a method for avoiding cross talk which may occur with such apparatus when a magnetic tape is recorded at one speed and played back at a different speed.
Devices which record and read magnetic tracks on a magnetic tape are well known in the art. Particularly well known are the devices and methods by which many parallel magnetic "tracks" representative of information are placed on and read from a magnetic tape. The magnetic tracks are recorded transversely to the direction of travel of the tape and continue in a side by side relationship along the full length of the tape.
To create the tracks on the tape, a magnetic head mounted on a rapidly rotating drum passes transversely to and in close proximity to the relatively slowly moving tape wrapped around said drum. The recorder produces a variable magnetic field in an air gap formed in the head. The magnetic field, in turn, creates the magnetic tracks on the tape. To play back the tape, a head is once again passed transversely across the tape in the same path as the track of the head upon recording. The magnetic track induces a voltage in the head.
A problem inherent in this method of operation is that when the tape speed is changed and the drum speed remains fixed, the angle at which the head transversely crosses the tape is changed. If, on playback, the tape speed is different from the tape speed at which the tape was recorded, a head sensing the tracks passes over more than one track on the tape. This "cross talk", in the case of video tape recorders for example, results in a distorted picture on the display. In the case of data storage, data error would occur as a result of the cross talk.
Many methods and devices have been invented to try to maximize the signal to cross talk ratio. Inventions such as disclosed in U.S. Pat. No. 4,731,659 issued to Kanni, attempt to eliminate cross talk by repositioning the head in order to compensate for the different head track angles during recording and playback. Another well known method, disclosed in U.S. Pat. No. 3,375,331 issued to Okazaki et al., is to dynamically reposition the tape in relationship with tape speed. A further well known method to limit cross talk is the introduction of an empty guardband between adjacent tracks as shown in U.S. Pat. No. Re. 31,078 issued to Segerstrom. However, all of these methods are not completely successful in eliminating cross talk. Furthermore, all of the methods require either mechanical movement of the head or substantially decreasing tape track density.
Thus it is an object of the present invention to provide a method and system for recording and playing back of a magnetic tape so that there is no cross talk during the playback of the magnetic tape. It is a further object of the present invention to provide a system and a method for substantially cross talk free playback such that track density is not substantially reduced, and dynamic repositioning of the heads and/or magnetic tape is not necessary.